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l/> no/I/I BUREAU OF MINES 

U SJ244 INFORMATION CIRCULAR/1990 



Longwall Automation: A Ground 
Control Perspective 

By Jeffrey M. Listak and Deno M. Pappas 




U.S. BUREAU OF MINES 
1910-1990 

THE MINERALS SOURCE 



Mission: Asthe Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9244 



Longwall Automation: A Ground 
Control Perspective 

By Jeffrey M. Listak and Deno M. Pappas 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 







Library of Congress Cataloging in Publication Data: 



Listak, Jeffrey M. 

Longwall automation : a ground control perspective / by Jeffrey M. Listak and 
Deno M. Pappas. 

p. cm. - (Information circular, 9244) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:9244. 

1. Longwall mining-Data processing. 2. Ground control (Mining). I. Pappas, 
Deno M. II. Title. III. Series: Information circular (United States. Bureau of 
Mines); 9244. 



TN295.U4 



[TN275] 622 s-dc20 [622\334] 



89-600351 
CIP 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Background 3 

Longwall ground control 4 

Problems inherent in longwall mining 5 

Pressure distribution 5 

Bumps 5 

Sloughing 5 

Cavities 5 

Geologic conditions 5 

Lithologic changes 7 

Structural relief 8 

Panel continuity 8 

Longwall layout and design 8 

Subsidence 9 

Multiple-seam mining 9 

Gate road pillar design considerations 9 

Longwall automation state of the art 9 

Roof supports 9 

Shearer automation 11 

Cutting horizon 11 

Face alignment 12 

Shearer position 12 

Component integration 12 

Constraints affecting longwall automation 13 

Ground control constraints 13 

Shearer 13 

Roof supports 14 

General constraints 14 

Bureau research 15 

Conclusions 15 

References 15 

ILLUSTRATIONS 

1. Number of U.S. longwall faces 2 

2. U.S. longwall production 2 

3. Cost breakdown of longwall components 2 

4. Representation of gamma-ray backscatter device 4 

5. Relationship between backscatter count rate and coal thickness 4 

6. Natural gamma radiation sensor 4 

7. Stress distribution profile in roof of typical longwall section 6 

8. Entry after bump occurrence 6 

9. Face sloughing 7 

10. Cavity or void along longwall face 7 

11. Fence diagram showing changing lithology above a longwall panel 7 

12. Stream valley profile 8 

13. Sandstone channel 8 

14. Fault- 9 

15. Trends in longwall shield controls 10 

16. Schematic showing two types of electrohydraulic systems for shield control 10 

17. Infrared technique used for shearer-initiated face advancement 13 

18. Rock on face conveyor 14 



TABLES 



Page 



1. Intelligent mining machines critical systems and technologies 11 

2. Status of shearer automation devices 12 

3. Summary of longwall automation monitoring devices 12 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


cps 


count per second min minute 


ft 


foot pet percent 


ft/min 


foot per minute s second 


in 


inch 



LONGWALL AUTOMATION: A GROUND CONTROL PERSPECTIVE 

By Jeffrey M. Listak 1 and Deno M. Pappas 2 



ABSTRACT 

This U.S. Bureau of Mines report describes the implications of in-mine ground control on the 
automated or remotely controlled operation of longwall mining equipment. Perhaps the greatest 
challenge to longwall automation researchers is the development of systems that will continuously 
function in the complex and unpredictable underground environment. The high degree of environmental 
variability, together with conditions brought about by the extraction process, makes complete automation 
of longwall mining a difficult task. The intellectual thought processes and split-second decision making 
required to avert disasters are lost when workers are removed from the face. As many mine operators 
can attest, minor problems, left unresolved, can eventually accumulate and lead to catastrophic 
consequences. The automation process will have to assess and manage various routine problems that 
are otherwise resolved through worker observation and experience. 



fining engineer. 

2 Research civil engineer. 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



In 1983, the number of operating longwall faces in U.S. 
mines peaked at 118 faces (i), 3 producing an estimated 
16 pet of the underground coal mined (figs. 1-2). By 1987, 
the number of faces had dropped to 101 (2), yet longwall 
mining produced over 21 pet of the underground coal 
mined. It is apparent that longwall productivity is increas- 
ing at an accelerating rate. Productivity statistics support 
this trend. In 1983, 64.7 face tons per worker day were 
being produced. By 1987, this number had doubled to 139 
face tons per worker day (3-4). This marked increase in 
productivity maybe attributed to improvements in longwall 
operation techniques, a leaner work force, and equipment 
sophistication. This trend will probably continue, espe- 
cially with the development of semiautomated and, in the 
future, completely automated longwall faces. 

With few exceptions, large mining companies have 
come to realize that the only way their mines can stay 
competitive in the domestic and world markets is to adopt 



Italic numbers in parentheses refer to items in the list of references 
at the end of this report. 




1979 1981 1983 1985 1987 1989 
Figure 1. -Number of U.S. longwall faces. 




longwall mining methods (5). Consequently, underground 
coal mining is moving toward high-production longwall 
systems. Currently, much time and effort are being spent 
in the development of semiautomated and automated 
production longwall systems. Although advances have 
been made, several key factors related to underground 
conditions need to be integrated into the overall design 
of an automated longwall system. Adverse geomechanical 
conditions are one of the most debilitating problems faced 
by longwall operators in terms of both production losses 
and accidents. A seemingly insignificant ground control 
condition could eventually interrupt the activity of the 
shearer, shields, or conveyor, any of which interruptions 
would halt the production of coal. Therefore, the success 
of an automated longwall system will depend on the ability 
of all the automated components to overcome geomechan- 
ical problems encountered during the course of longwall 
panel extraction. The initial capital expenditure required 
for providing face support to a longwall system emphasizes 
the necessity of coping with ground control conditions. 
Figure 3 shows that over 70 pet of the total cost of long- 
wall face equipment goes toward the cost of the longwall 
supports. 

It is the intent of this report to review the state of the 
art in longwall automation and to identify ground control 
conditions that may affect the operation of an automated 
longwall system. This research is in support of the 
U.S. Bureau of Mines goal to develop longwall systems 
that allow for continuous and autonomous operation. 



I983 I984 I985 I986 

Figure 2.-U.S. longwall production. 



I987 




Figure 3.-Cost breakdown of longwall components. 



BACKGROUND 



The need for longwall automation was first recognized 
and addressed by the British in the early 1960's, with the 
Remotely Operated Longwall Faces (ROLF) program of 
the National Coal Board (NCB). This ambitious program 
attempted to automate the longwall system as a whole; 
however, at the time, mechanization was not developed to 
the level of reliability necessary to make automation fea- 
sible (6). By the end of the 1960's, the program was ter- 
minated; the lesson learned was that, for an automation 
program to be successful, a step-by-step approach of 
looking at each longwall component separately is required. 
The results of this work laid the foundation for the de- 
velopment of electrohydraulically controlled supports, 
which utilize an electrically activated compacted solenoid 
valve for controlling the hydraulic systems for the roof 
supports (7). Although it took nearly 20 years for the 
electrohydraulically controlled supports to become com- 
mercially available, this technology was a major contri- 
bution to automating the longwall mining system. A de- 
tailed description of electrohydraulic control of supports 
is presented later in this report, in the section "Roof 
Supports." 

Another NCB initiative, known as the Advanced Tech- 
nology Mining (ATM) program, was aimed at improving 
production and productivity on longwall faces by consol- 
idating the reliability of proven equipment and moving in 
small steps toward automation (6-7). Out of this program 
developed two types of nucleonic sensors for detecting the 
coal-roof interface as the shearer is cutting. The first 
nucleonic probe basically consists of a transmitter with a 
radioactive source, usually cesium- 137, and a receiver that 
detects the radiation. The gamma-ray backscatter device 
consists of a gamma radiation source that is directed onto 
the roof material and a method of radiation detection to 
monitor the backscatter ed radiation from the roof (8). As 
shown in figure 4, the sensor, known as the gamma-ray 
backscatter sensor, is enclosed in a housing positioned 
near the surface to be measured (9). A narrow beam of 
gamma rays is directed against the rock and is subject to 
scattering processes and attenuation. The amount of in- 
cident rays that are backscattered is inversely proportional 
to the density of the rock type. Figure 5 (10) illustrates 
the relationship between backscatter and coal thickness. 
In order for the probe to work accurately, it has to be in 
close contact with the surface of the roof; an air gap or 
void significantly reduces the accuracy of the results. In 
addition, the presence of a radioactive source in the probe 
was a major detriment toward further development. From 
these disadvantages evolved the second sensor, referred to 



as the "natural gamma background sensor" (fig. 6/1). This 
system allows the coal thickness above the shearer to be 
measured using the background radiation naturally emitted 
by most strata composed of shale (6). Radiation from the 
roof rock is attenuated exponentially by any coal left in 
place (8, 11-12). The coal thickness can be measured 
using an empirically determined attenuation curve, shown 
in figure 6B. Voids and air gaps are not a problem with 
the natural gamma background sensor; however, the probe 
does not work for nonshale strata. 

By 1980, the second probe was refined and mounted on 
the shearer along with a computer-based control data 
transmission system and was termed the "System 70000" 
(6). This system has recently become commercially avail- 
able as the Machine Information Display and Automation 
System (MIDAS) 4 (13). 

At the same time the NCB's ATM program was being 
developed, the U.S. Bureau of Mines began its longwall 
automation program focusing on shearer guidance sensors 
to detect the seam roof or floor interface. Three types of 
systems were developed (14): 

• Vertical Control System (VCS).-VCS sensors in- 
clude coal interface detectors, last cut height sensors, and 
interface connections to the shearer drum arm actuators 
and controls. 

• Face Alignment System (FAS) -FAS sensors include 
coal face alignment sensors measuring the yaw and roll, 
and interface connections to the roof support and shearer 
tilt actuators. 

• Master Control System (MCS).-MCS components 
include the software, controls, and information display 
necessary for operators to control the longwall operation 
from the headgate location. 

When the U.S. Department of Energy (DOE) took over 
some of these Bureau projects, the VCS was field tested. 
Unfortunately, ?}>& trial run was discontinued owing to very 
severe ground control conditions. The DOE continued to 
work on surface recognition and coal depth measuring 
sensors. These sensors and detectors included nucleonic, 
electromagnetic, radar, and machine vibration types. 

Recent developments in longwall automated systems 
are presented in the section "Longwall Automation State 
of the Art." 



Reference to specific products does not imply endorsement by the 
U.S. Bureau of Mines. 



Rock 



0M&0MMXM&Mi 



A 



Scattering and attenuation 
of gamma rays 



Protective mounting 



Detector - 



Arm 

on shearer 



Source surrounded 
by heavy metal 
shielding 
mounted 






//»'•' 



Shale 
( radiation source) 

Coal 

( absorber) 



Sensor to 
detect roof 
contact 



Figure 4. -Representation of gamma ray backscatter device. 
[Adapted from Wood (9)] 



Gamma 
detector 




£ 400 


i i 


i i 


o 






LU 






b 300 


— 


/ - 


< 






rr 






£ 200 




- 


3 






O 






° 100 


i i 


i i 



300 



2 4 6 8 

COAL THICKNESS, in 



10 




Figure 5.-Relationship between backscatter count rate and 
coal thickness. [Adapted from Clayton (10)] 



2 4 6 8 10 12 14 

COAL THICKNESS, in 



Figure 6.-Natural gamma radiation sensor. A, Schematic 
representation; B, calibration curve. [Adapted from Tregelles 
(12)] 



LONGWALL GROUND CONTROL 



Longwall ground control can be defined as a method 
of managing an unstable underground opening resulting 
from the redistribution to roof support elements of forces 
induced in the roof, rib, and floor during the mining pro- 
cess. Longwall mining ground control is of paramount 



importance because effective strata control is to the coal 
operator's advantage in mining the coal, while ineffective 
control is to the coal operator's detriment in that bad 
conditions could result in the abandonment of the mine 
section or entire mine. 



There are three major aspects of ground control that 
need to be considered when addressing the longwall auto- 
mation program: the ground control problems inherent in 
the longwall method of mining, the ground control prob- 
lems stemming from the changing geologic conditions 
found in all underground mines, and the unique ground 
control problems related to the different designs and 
layouts of longwall panels. 

PROBLEMS INHERENT IN LONGWALL MINING 

The longwall system of mining coal in 665- by 5,000-ft 
panels (average dimensions) with the roof caving behind 
the face and several gate road entries separating the panels 
presents ground control problems unique to underground 
mining. The total extraction of a large block of coal 
causes high stress concentrations, along the face and in the 
gate roads, and poses some difficult ground control prob- 
lems depending upon the competency of the immediate 
and main roof rock. The following areas need to be con- 
sidered for the successful design and implementation of 
an automated longwall system. 

Pressure Distribution 

When the mine entries are developed, the preexisting 
stress equilibrium is destroyed because of the extraction of 
the coal. The weight of the overburden previously sup- 
ported by the excavated coal must now be carried by the 
neighboring solid coal in the panel and the pillars. These 
regions, where the vertical pressures exceed the average 
overburden pressure, are referred to as the "abutments" 
(15). Figure 7 shows a typical pressure redistribution 
pattern including the front and side abutment and the 
pressure buildup in the gob. In the overall design of an 
automated longwall system, the pressure distribution along 
the face and gate roads needs to be examined for the 
following reasons: (1) pillars must be sized to support 
anticipated loads, (2) roof falls are inevitable under some 
extremely poor roof conditions and the automated system 
must be protected from such events when they occur, and 
(3) system flexibility is required to respond to changing 
conditions. 

Bumps 

In some longwall mines the problem of mountain 
bumps or pressure bursts has become a major ground 
control concern. It is thought that bumps or bursts are 
triggered by the excessive loading of the pillars because 
of the massive overburden, which exceeds the bearing 
strength of the coal and results in a sudden and violent 
rupture of the face or supporting gate road pillars. Al- 
though bumps can occur in all types of mining methods, 
this phenomenon is usually more pronounced in deep 
mines with massive lithologic members present in the 
overburden (16) and to some degree is affected by the 



physical properties of the coal being mined. Some bumps 
are so large they generate seismic waves that register 
between 3 and 4 on the Richter scale, cause tremors on 
the surface, and result in serious consequences under- 
ground (fig. 8). Research into bump phenomena con- 
tinues, and although it is thought that the proper design 
of gate road pillars can guard against or control bump 
occurrence, a solution does not appear imminent. 

Sloughing 

One of the repercussions resulting from the weight of 
the overburden being transferred to the gate road pillars 
and longwall face is the sudden failure of portions of the 
outer skin of the pillar or longwall face (fig. 9). Although 
most rib sloughing incidents are minor, they may inhibit 
the movement of coal out of the headgate or halt the 
production of coal because large pieces of coal or rock fall 
and choke the conveyor at the stage loader. Consequently, 
these occurrences would have to be dealt with when 
considering an automated longwall system. 

Cavities 

During the longwall mining process, highly fractured 
roof may be encountered, resulting in falls between the 
face and support canopy tips (fig. 10). Consequently, these 
cavities cause the support to lose contact with the roof. If 
this condition persists, large roof voids develop above the 
longwall supports, disabling their movement and halting 
production. Correction of this problem often requires that 
miners climb on top of the supports and construct cribs 
to reestablish roof contact. If this condition occurs, it 
defeats the purpose of having an automated longwall since 
the miners are subjected to hazardous roof while con- 
structing the cribs. Therefore, it is imperative that mea- 
sures be taken to prevent the occurrence of cavities. 

GEOLOGIC CONDITIONS 

Whereas the first set of conditions affecting ground 
control is a result of the mining-induced stresses during 
coal extraction, the second set of conditions is a result of 
the geologic features already present in the coal measure 
rocks. These naturally occurring phenomena consist of 
major and minor geologic features and structures, includ- 
ing changing lithology, structural relief, faults, sand chan- 
nels, slickensides, seam height, clay veins, joints, kettle- 
bottoms, seam undulations, and soft floor. The occurrence 
and implication of these features can vary greatly from 
region to region and often within the same coalbed and/or 
mine. The effects of these geologic features are magnified 
by the mining-induced stresses mentioned previously. 
These geologic conditions need to be confronted for 
longwall automation to be successful in the U.S. mining 
industry. 



Direction 
of mining 



Head 
entry 




41 ! i** 



Figure /.-Stress distribution profile in roof of typical longwall section. [Adapted from Whittaker (75)] 




Figure 8.-Entry after bump occurrence. 




Figure 9.-Face sloughing. 



-Cavity 



~^ — — /s n° o /> o» 
rlmmediate roo1-=-^j&frSm0osCp 




Figure 10.-Cavity or void along longwall face. 



Lithologic Changes 

Over the extent of a mine, it is not unusual to encoun- 
ter coal measure rocks that vary in type, property, and 
thickness (fig. 11). Variation in lateral continuity of roof 
and floor strata present numerous problems to an auto- 
mated system. As roof and/or floor rock gradually grade 
into other rock types, adjustments to equipment are re- 
quired as the face advances. For example, floor rock that 
had sufficient bearing strength at the beginning of a panel 
can become soft as the panel is retreated and thus pose 
serious problems with the performance of face supports. 
Similarly, the immediate roof can change from being very 
uniform and stable to being fractured or friable and, sub- 
sequently, prohibitive to mining and damaging to equip- 
ment. In extreme cases, massive sandstone roof, which 
does not readily cave, can create conditions that can lead 
to excessive loading and, eventually, pressure bursts 
(bumps). 

As the dimensions of longwall panels continue to in- 
crease, system flexibility will be required to control the 
different types of strata encountered throughout the length 
of the panel. 








1,000 



Scale, ft 



LEGEND 



H2J Sandstone 
Shale 
Coal 



IShaly limestone 
Limestone 



Figure 11. -Fence diagram showing changing lithology above longwall panel. 



Structural Relief 

Overburden thickness often varies over the length of 
longwall panels. Topographic relief from to 2,000 ft 
over short horizontal distances (less than 1 mile) is not 
uncommon in drift mines located in mountainous regions. 
Extreme changes in overburden must be considered prior 
to selection of support equipment so that proper capaci- 
ties of both face and supplemental support can be imple- 
mented to control changes in cover loads. 

Stream valleys located above coal reserves also create 
a unique set of stress-related problems. Topographic 
relief, formed by years of normal erosion of weak, flat- 
lying sediments, creates a zone of lateral compression, as 
shown in figure 12 (17). These premining stress field 
concentrations beneath stream valleys are often the cause 
of roof failure brought about by the persistent shearing of 
the rock at the rib-roof interface. This type of roof failure, 
commonly known as cutter roof, is nearly impossible to 
support by conventional support methods. 

Panel Continuity 

Successful automation of longwall mining equipment is 
very promising because of the repetitive nature of the 
extraction process inherent in the longwall mining method 
and the fact that most coal seams are generally homoge- 
neous and flat lying. However, geologic anomalies such as 
faults, clay veins, sand channels, kettlebottoms, seam un- 
dulations, interbedded seams, variation in seam height, 
and coal quality create challenges for developers of au- 
tomation technology. For instance, sand channel and clay 
vein intrusions can occur periodically without warning and 
interrupt the otherwise uniform interface between the coal 
and the roof (fig. 13). In general, operators cut through 
the intrusion in order to maintain uniform roof height. On 
the other hand, a sensor that detects the coal-rock 
interface may attempt to follow the irregularity, causing 
inconsistencies in the roof and subsequent problems for 
roof supports. In the Western United States, the presence 
of faults can signify a sudden and complete loss of coal 
reserves, requiring complex mining adjustments (fig. 14). 
Fortunately, retreat longwall mining offers an advantage in 
that many of these features are recognized during the 
development of the longwall panel and prior to the setup 
of the face equipment. In addition, some of these 
conditions can be detected and managed with various 
devices (i.e., remote sensing and electromagnetic 
instruments). 

LONGWALL LAYOUT AND DESIGN 

Mines in existence prior to the use of longwall methods 
were limited subsequently in the use of longwall mining 
because of Federal subsidence laws, which restrict the 



complete extraction of coal beneath surface structures 
(buildings, highways, cemeteries, etc.). This means careful 
planning, and oftentimes interruptions, of panel layout and 
projections to avoid populated areas. More recently, coal 
mines are being designed to include longwall practices, 
and as a result, wider and longer panels (super panels) are 
being developed away from populated areas. Super long- 
wall panels have widths greater than 900 ft and lengths 
greater than 10,000 ft. The motivation behind this trend 
in super longwalls is to maximize the production of coal 
and minimize the long-term effects of surface subsidence. 
Mining larger panels and mining continuously using an 
automated longwall face could significantly increase pro- 
duction. For instance, as panel dimensions increase, the 
frequency of setup and transfer of equipment, a very time 
consuming process, decreases. However, at some point, 
equipment transfer to the next panel is required when the 
current panel is depleted of coal. Equipment recovery 
areas require special preparation near panel recovery 
points and have yet to be addressed by the automated 
process. 



Zone of high 
lateral compression 




LEGEND 

Resultant of 
overburden stresses 



400 

I 



Approx scale, ft 



Figure 12.-Stream valley profile. 




Figure 13.-Sandstone channel. 




Figure 14.-Fault 



Subsidence 

Longwall mining allows the roof to cave quickly, uni- 
formly, and completely, thus significantly reducing the 
long-term damaging and unpredictable effects of surface 
subsidence usually associated with room-and-pillar mining. 
Furthermore, a faster advance rate lessens surface defor- 
mations by reducing the magnitude of the inclination, 
curvature, and tension and compression zones ahead of the 
face, thus enabling the surface to settle with less damaging 
effects. An automated longwall system will allow for a 



faster rate of advance and tend to reduce the damaging 
effects of subsidence. 

Multiple-Seam Mining 

As mines expand, mining below or above previously 
mined-out areas or presently active mines will become 
more common, especially in the eastern U.S. coalfields. In 
West Virginia alone, there are over 50 minable seams in 
multiple-seam configurations (18). Analysis of ground 
control multiseam problems shows that factors contributing 
to interactions may be classified into variables that are 
fixed by the geologic environment and those that depend 
on engineering design (79). Although with proper design 
considerations automated longwall mining can be realized 
in multiple-seam situations, caution must be exercised 
when designing gate road and panel projections. 

Gate Road Pillar Design Considerations 

Another problem related to panel layout is that the 
design of gate roads often depends upon the prevailing 
roof conditions in a particular area during development, as 
well as supply, ventilation, and escapeway requirements. 
These conditions influence the number of entries and the 
type of pillar design, either desired or deemed necessary, 
in longwall gate roads. However, the best pillar configu- 
ration for developmental purposes does not always coin- 
cide with the most effective ground control methods for 
maintaining entry stability during longwall retreat mining. 



LONGWALL AUTOMATION STATE OF THE ART 



Longwall mining is a repetitive process and one con- 
ducive to automation; however, the extremely harsh un- 
derground environment and unpredictable geologic con- 
ditions have seriously impeded automation progress. In 
general, the health and safety benefits that can be achieved 
from an automated longwall process are the removal of 
personnel from the hazards associated with immediate face 
exposure (i.e., dust, roof falls, noise, pinch points, etc.), 
while the economic benefits are a higher quality product, 
lower maintenance costs (less bit wear), increase in speed 
of operation, and better personnel utilization. 

Various avenues of automation technology are being 
explored. Peterson (20) believes that the currently existing 
longwall component configuration is not the best basis for 
automation and that if a longwall system were designed 
specifically for automation, a differently designed system 
would be produced. However, other researchers (11, 21- 
24) and manufacturers are working within the currently 
available equipment specifications. As previously stated, 
automation of a longwall face is not a new concept; how- 
ever, initial attempts failed because the technology was 
not available to make complete automation of individual 
components an integrated, error-free process. Today, 
positive technological advancements have been made in 
automation of individual components, particularly shield 



supports. Although advances in shearer automation have 
been realized, no current breakthrough can be reported. 

ROOF SUPPORTS 

Shield supports now make up 98 pet of the longwall 
face support installations in the United States (7). Two 
major groups of shield manufacturers have clearly influ- 
enced the development of shield support application in the 
United States. The basic structural design parameters of 
the caliper and lemniscatic shields were introduced by 
West German manufacturers, and the hydraulic and later 
electrohydraulic auto-control concepts were innovations of 
the British (25). Perhaps the most significant advance 
toward automation has come from the development of 
electrohydraulic control systems for shield supports. The 
earlier pilot-operated valve systems have been superseded 
by electrohydraulic control systems. Electrohydraulic 
shields were introduced in the United States in 1984. 
Since then, the technology has been readily adopted by 
longwall operators (fig. 15). Initially, the move to an 
electrohydraulically controlled support was made because 
electrohyraulics provided the ability to move a group or 
batch of supports from one dustfree location. Also, these 
types of supports were able to achieve a uniform set 



10 



120 
100 
80 
60 
40 
20 



KEY 
Manual 
Electrohydraulic 
Shearer initiated 




1985 



1986 



1987 



1988 



1989 



15.-Trends in longwall shield controls. 



pressure against the roof and a much faster cycle time (6 
to 10 s per support). Base lifting to overcome soft floor, 
which had been available on hydraulically controlled sup- 
ports, has been successfully incorporated into the electro- 
hydraulic systems. 

In general, the four major components of an electro- 
hydraulic control system are (1) the hydraulic directional 
control valves, (2) the solenoid valves, (3) the electronic 
control unit, and (4) the headgate computer. General 
schematics of the two systems currently in use are shown 
in figure 16 (26). The major difference in the systems is 
the function of the microprocessor contained in each elec- 
tronic control unit. In system A, the signals are sent to a 
master computer for interpretation. The commands are 
then issued to cycle the next support in the batch. Each of 
the supports in the system is dependent on the headgate 
computer for cycling. In system B, the microprocessor on 
each shield interprets signals and relays commands to the 
next support to initiate the support cycle. In this system 
each support is "smart," thus eliminating the need for a 
master computer. 



System A 



Power 
supply 



Headgate 
computer 




Master cable 



KEY 
E Electronic control unit 

H Hydraulic control valve block 

S Solenoid valve 



System B 




Master and slave cable 

Figure 16.-Schematic showing two types of electrohydraulic systems for shield control. [Adapted from S. Peng (26)] 



11 



Initially, electrohydraulic control units were easily in- 
capacitated by dust and moisture migration into the unit's 
electronics. As a result, supports, although equipped to 
operate in batch mode, were being activated singularly 
from an adjacent support. However, reliability has greatly 
improved and batch advance of supports is now the rule 
rather than the exception for installations utilizing electro- 
hydraulic control. Support manufacturers agree that, al- 
though there is room for improvement, the dependability 
problems first encountered with electronic components 
have been sufficiently overcome so that today few, if any, 
support installations are purchased without electrohydrau- 
lic control technology. 

The successful application of support advance technol- 
ogy is currently evolving into what eventually will become 
a completely automated longwall face. Currently, the use 
of automatic support advance by shearer initiation is 
gaining in popularity, and there are several installations 
with this capability. However, the use of shearer initiation 
for the automatic advance of face supports is at the same 
stage as that of batch control several years ago (i.e., mines 
have the technology but are not utilizing it). Two mines 
that have installed shearer-initiated support advance are 
reluctant to use it, for different reasons. One operator 
recognizes its benefits, but mine officials fear that miners 
are threatened by its presence; they are therefore allowing 
for an extended training and breaking-in period. The 
other operator does not see production advantages over its 
current method of mining. In general, operators are able 
to meet their contractual obligations by employing meth- 
ods that do not require the added expense of automated 
equipment. In addition, some operators are reluctant to 
adopt technology that is not yet fully proven and thus will 
wait for the development of more reliable systems. 

SHEARER AUTOMATION 

To manage the complex conditions that will be encoun- 
tered underground, it is not enough for shearing machines 
to be automated. Instead, machine "intelligence" will be 
required, not only to perform normal operations, but to 
detect, interpret, and respond to possible changes in nor- 
mal operations (many of which are ground control relat- 
ed). Computerization has provided the integration of once 
stand-alone electronic components, enabling on-board 
intelligence. With the existence of a proven interface on 
the machine, the addition of feedback sensors can allow 
for the use of on-board intelligence (27). Some of the 
early technical endeavors that were the basis for the cur- 
rent state of the art are presented in the "Background" 
section of this report. A synopsis of the Bureau's robotics 
and automation research program, by Schnakenberg (28), 
lists the critical path technology categories for intelligent 
mining machines (table 1). 

Three important areas must be addressed before 
shearer automation can become practical (9): (1) cutting 
horizon, (2) face alignment, and (3) shearer position. 



Table 1 .-Intelligent mining machines critical systems 
and technologies (28) 



System 
Basic machine 



Computer systems 



Machine control 



Guidance systems 



Coal interface 
detection. 



Diagnostics 



Planning 



Dominant components and issues 

Mechanical, electrical, and hydraulic 
systems. 

Processor boards, operating system; 
communication networks for real- 
time, multiple-processor operations. 

Position, electrical, and hydraulic 
sensors; computer data acquisition, 
closed-loop control algorithms, and 
command language. 

Position and heading sensors and 
systems, obstacle and mine rib 
detection and registration to map 
data, computer software for data 
fusion and filtering. 

Coal-strata properties and differences, 
multidisciplinary systems for real- 
time sensing of cutting position, 
artificial intelligence data interpreta- 
tion. 

Sensors of machine condition, expert 
system interpretation and analysis, 
human interfacing. 

Data interpretation, knowledge repre- 
sentation, artificial intelligence, and 
human interfacing. 



Cutting Horizon 

The automated shearing machine will replace the ma- 
chine operator and therefore must execute the functions 
a human operator would perform using sensory perception. 
Machine operators maintain horizon control by observing 
the position of the cutting drums and making adjustments 
according to the coal-rock interface or the desired thick- 
ness of the cut. Shearing can take place at the coal-rock 
interfaces of the roof and floor, or coal may be left un- 
mined either at the roof or at the floor depending on the 
competency of the rock or the seam height desired. Sen- 
sors have been developed to maintain cutting horizon in a 
variety of ways; however, the primary purpose is either to 
differentiate between coal and rock or to determine the 
thickness of coal left at the roof. 

These sensors are in various stages of development, and 
thus far, no one sensor has been a total success. The 
sensitized pick transducer uses pick force to guide the 
ranging drum along the correct horizon by maintaining a 
continuously updated hardness profile of particular bands 
within the coal seam (6). The vibration transducer dis- 
criminates between the coal-rock interface, while other 
sensors, such as the natural gamma radiation, doppler 
radar, and electromagnetic wave sensors, can be used to 
detect coal thickness left at either the roof or floor. Main- 
taining horizon control at the floor presents problems for 



12 



many of the sensors because there is no way to mount 
sensors on either the shearer or face conveyor to transmit 
signals into the floor. The type of sensor utilized on a 
particular shearing machine would depend on the appli- 
cation desired or on ground control conditions either pres- 
ent or expected. 

Face Alignment 

During the retreat of a longwall panel, it is not unusual 
for face equipment to gradually wander out of alignment 
(i.e., the longwall face is no longer perpendicular to the 
head and tailgate entries). To correct this occurrence, 
shearer operators make angle or wedge cuts to straighten 
the face. British Coal is developing sensors to maintain 
face alignment (24). Transducers on the support advance 
rams and distance measurements using a cord transducer 
located in the gob are two promising face alignment 
techniques. 

Shearer Position 

As the shearer traverses the face, cutting coal, the 
shields are advanced to support the exposed roof, followed 
by the push or advance of the conveyor, which in turn 
enables the cycle to begin over again. Since the latter two 
operations (shield advance and conveyor push) depend 
upon the position of the shearer along the face, timing 
during the cycle is critical. An automated system must 
continually monitor shearer position so that effective ad- 
vance of supports and conveyor can be performed in prop- 
er sequence. 

Two types of systems have been developed and suc- 
cessfully implemented to monitor shearer direction and 
position along the face. One system employs the shearer 
haulage drive gear as an odometer. As the drive gear 
rotates, pulses are generated, incremented, and relayed to 
a central control unit in the headgate at regular distance 
intervals along the face. The control unit is linked to the 
roof supports and, by recognizing the position of the 
shearer, is able to relay commands to activate the support 
advance and conveyor push at a preset distance behind the 
shearer. 

The other system utilizes an infrared transmitter 
mounted on the shearer and receivers located on each of 
the supports (fig. 17). As the shearer moves along the 
face, the infrared signals are received by the supports and 
relayed to the headgate control unit. The control unit 
defines the position and direction of the shearer and con- 
veys commands to initiate support advance and, subse- 
quently, conveyor push. 

A summary of shearer system readiness, as compiled by 
one automation researcher, is shown in table 2 (21). 

COMPONENT INTEGRATION 

The various sensors being developed for an integrated 
automated system are presented in table 3. The continual 
development and reliability of individual components along 



with the introduction of microprocessors (computerization) 
has enabled some degree of successful automation. The 
successful interfacing of supports and shearer has been 
achieved through the use of electrohydraulics and shearer 
position technology. Although advances are being made, 
the shearer remains the final obstacle in achieving total 
longwall automation. 



Table 2 -Status of shear automation devices (21) 



Ready 



Partially complete 



New work 



Shearer face location sensor. 

Pitch and roll sensors. 

Coal thickness sensor. 

Computer and electronics. 

Digital communications. 

Error condition reporting. 

Ability of equipment to with- 
stand environment. 

Auto floor slave and robotic 
roof control. 

Roll stability on floor cut. 

Evaluation vibration sensor. 

Addition of high-performance 
coal thickness data. 

Cut control using coal thick- 
ness data. 

Shearer turnaround control. 

Integration tests with self- 
advancing shields. 

Optimizing shearer and pan 
designs. 

Maintenance procedures, doc- 
umentation, and training. 



Table 3.-Summary of longwall automation 
monitoring devices (9) 



Parameter monitored 
and device used 



Function 



Cutter position: 

Odometer Measure shearer position along face. 

Infrared transmitter ... Do. 

Cutter roll: 

Tilt transducer Measure shearer roll. 

Inclinometer Do. 

Horizon control: 
Natural gamma Measure roof coal thickness. 

radiation sensor. 
Doppler radar sensor . . Do. 

Sensitized pick force Differentiate between coal and non- 

sensor, coal. For locating interface or for 

horizon control by using marker 
bands. 

Vibration sensor Measure roof coal thickness, locate 

coal-rock interface. 

Memory pass Recall preprogrammed cut profile to 

be duplicated on successive 
shearer passes. 
Measure roof coal thickness. 



Electromagnetic 
wave. 
Face alignment: 
Cord transducer 



Advance ram 
transducer. 



Measure straightness of face, de- 
termine angle of face with road- 
ways, and measure advance of face. 

Measure advance of face. 



13 







Figure 17.-lnfrared technique used for shearer-initiated face advancement [Adapted from S. Peng {26)] 



CONSTRAINTS AFFECTING LONGWALL AUTOMATION 



The authors visited several longwall operations through- 
out the United States to observe the various degrees of 
automated face equipment in use and to discuss with op- 
erators constraints that may limit the further development 
of automated operations. In general, constraints affecting 
equipment (shearer, roof supports, conveyors) as well as 
the mine atmosphere (ventilation, dust, methane) emerge 
as impediments to an automated system. Ideally, auto- 
mated face equipment will isolate mine personnel from 
face hazards while providing significant increases in pro- 
duction; however, compliance with regulations regarding 
dust and methane concentrations must still be enforced to 
guard against catastrophic mine explosions. In addition, 
large production capacities at the longwall face are limited 
by the capacity of the conveyor, as well as outby haulage 
systems that transport coal to the surface. 

Since the two major active components of a longwall 
system are the shearer and roof supports, the following 
sections present ground control and other general con- 
straints to automation that affect these two components. 

GROUND CONTROL CONSTRAINTS 

Shearer 

When personnel are present along the face, problems 
that arise are immediately diagnosed and remedied. Con- 
sequently, by the routine recognition and correction of 
small problems, major production-stopping problems are 
averted. The key to successful automation will be the 



ability of the equipment to overcome problems routinely 
associated with daily operations. 

Frequent production delays due to shearer stoppages, 
not related to equipment failure, along the face can usual- 
ly be attributed to rocks on the face conveyor on the tail- 
gate side of the shearer that are too large to pass between 
the shearer and conveyor. These rocks usually fall from 
the unsupported roof span between the face and the sup- 
port canopy and must be broken into smaller pieces by a 
sledgehammer in order to pass beneath the shearer (fig. 
18). Large rocks that fall on the headgate side of the 
shearer also have to be broken to pass from the face con- 
veyor to the stage loader. 

Events such as pressure bursts (bumps) and gate road 
roof falls occur with less frequency but are major pro- 
duction hindrances and, in the case of bumps, pose serious 
safety hazards. The magnitude of a bump is dependent 
upon the amount of stored energy in an area. In one 
mine, a large bump damaged shield supports so severely 
that extreme measures (blasting) had to be employed to 
remove one-third of the supports from the face area. In 
another case, a bump near a shearer twisted and bent the 
ranging arms of the shearer, requiring costly repairs (about 
$1 million). Some western U.S. longwall faces frequently 
experience small coal bumps along the face as the shearer 
cuts. These bumps hurl fragments of coal out into the 
walkway where longwall crews work. Miners protect them- 
selves from flying debris by walking between support legs 
or by hanging lengths of belt from shield supports in bump 
areas. The shearer operator, however, must remain in the 
walkway unprotected. 



14 




Figure 18.-Rock on face conveyor being broken up into 
smaller pieces with sledgehammer. 



Roof falls in gate road entries also inhibit production 
and provide the potential for the development of danger- 
ous situations. Tailgate falls block aircourses for venti- 
lation, which can lead to the buildup of dust and methane 
concentrations. Extensive maintenance is required to 
reestablish airflow for face ventilation. 

Roof Supports 

Successful control of the roof and problem-free cycling 
of longwall face supports are the basic principles required 
for a productive longwall. However, there are situations 
that require the constant attention of longwall personnel to 
prevent support-related production stoppages. The most 
common ground-control-related occurrence that hinders 
support functions is soft floor. Floor rock is often com- 
posed of shale or fire clay. The properties of the rocks 
vary and may be adversely affected by water. On compe- 
tent, level floor rock, supports slide forward during the 
advance cycle. However, support bases penetrate soft rock 
and dig into the floor, which requires that the base be 
lifted out of the depression. 

The reaction of the floor to the support is also depen- 
dent upon support operation. For better control of the 
roof, support operators bias support force toward the tip 
of the canopy. This shifts the immediate roof loads toward 
the front of the base and reduces the bearing area of the 
support base, causing the base to break and penetrate the 
floor, resulting in advance problems or, worse, support 
instability. Support manufacturers have designed support 
bases to overcome soft floor and, as previously stated, have 
successfully incorporated this technology into the automatic 
advance of supports. 



A situation of seemingly less importance but one that 
requires constant maintenance by face crews is the pushing 
or plowing of debris (loose coal and floor rock) ahead of 
support bases. Continuous shoveling of this material is 
required to allow for unimpeded advance of supports. 

Another roof-support-related problem is that support 
loads across the face may not be uniform and may require 
different setting pressures depending upon the location of 
increased load. Nonuniform loading is determined through 
observation, and support setting pressure is adjusted ac- 
cordingly. By incorporating support leg pressure trans- 
ducers into the automated system, support leg pressures 
can be continually monitored and adjusted automatically to 
achieve an appropriate setting pressure. 

Shearer tram times across the face continue to increase 
and are currently rated at about a 40- to 45-ft/min maxi- 
mum (29). On a 600-ft-wide face utilizing 120 shield sup- 
ports, a shearer, moving at 40 ft/min, would take 15 min 
to cut one direction. The shields, cycling at 10 s, would 
take 20 min to advance the entire face. This procedure 
would require the shearer to wait 5 min every cutting pass. 
However, by cycling shields simultaneously, advance times 
are dramatically increased. The ground control impli- 
cations of moving two or three adjacent supports at the 
same time are obvious; however, manufacturers have de- 
signed electrohydraulic controls to systematically advance 
nonadjacent supports simultaneously and thus allow for 
roof control between advancing supports. 

GENERAL CONSTRAINTS 

Other problems facing complete longwall automation, 
not necessarily related to ground control, must also be 
considered. In general: 

• More complex machinery requires more mainte- 
nance to keep it functional; 

• More highly skilled personnel will be required to 
perform remedial maintenance measures when equipment 
malfunctions; 

• The type of sensors utilized may depend upon the 
specific application desired or geologic conditions present 
or expected; 

• Electronic devices will have to be rugged to en- 
dure continual abuse from machine vibration, dust, and 
moisture; 

• Control and articulation of cutting drum cowls has 
not been successfully accomplished; 

• Integration of sensor detection, computer inter- 
pretation, and machine response must occur nearly 
instantaneously; 

• Work force attitudes toward automation are 
negative. 



BUREAU RESEARCH 



15 



The Bureau's research has been directed to develop 
automation technology for continuous mining machines, 
because 75 to 80 pet of underground production (room- 
and-pillar) is mined by continuous miner and because 
continuous miners provide development for longwall re- 
treat, shortwall, and highwall mining. The Bureau is ac- 
tively involved in the development of intelligent machine 
control technology. The major thrust of the research is 
concentrated in the area of coal interface detection (CID) 
and machine guidance. Several promising CID techniques 
are currently being investigated, including machine vibra- 
tion and adaptive signal discrimination, natural gamma 
radiation, thermography, X-ray florescence, and radar (22). 
The subsystems being developed can be utilized individ- 
ually in the near term as aids to remotely controlled 



continuous mining. These sensors, in turn, can be 
extended to automation of longwall mining systems. 

Bureau research is divided into three categories: (1) 
a fundamental or core program of research that explored 
and developed the fundamental knowledge and hardware, 
(2) an applied research program that brings the pieces 
or integrated systems to a demonstrable, reliable, and 
fieldworthy stage, and (3) ad hoc solutions to mining in- 
dustry needs (28). 

The Bureau is committed to research that will increase 
coal extraction efficiency by increasing machine availability 
time and utilization. This goal will be accomplished by 
developing and integrating systems that will allow for con- 
tinuous and autonomous operation of mining machines. 



CONCLUSIONS 



Although most technologically advanced automated 
machinery operates in a highly controlled environment, this 
is not possible in the underground coal mine. Therefore, 
the success of an automated longwall system hinges on the 
ability of the system to handle routine environmental con- 
straints, including dust and methane, and ground control 
limitations that all mines cope with on a day-to-day basis. 
Ground control constraints include mining-induced stresses 
inherent in longwall mining (e.g., bumps, sloughing, cav- 
ities), geologic conditions (e.g., lithology, structural relief, 



geologic anomalies), and longwall layout (e.g., panel size, 
setup, recovery room) that must be considered and incor- 
porated into the overall operating design of an automated 
longwall. 

While the various components needed to automate a 
longwall face are at various stages of maturity, it is evident 
the mining industry will gradually integrate all of these 
components into a totally automated longwall system such 
that they are designed to function within the environmental 
limitations of an underground mine. 



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